MALDI mass spectrometry is a highly sensitive method for the detection and identification of biological macromolecules such as peptides, proteins and nucleic acids. In this method the analytes are ionized by bombardment with laser radiation in a vacuum chamber. Some other ionization methods applicable to biopolymers are carried out at atmospheric pressure, e.g., electrospray ionization (ESI). In general, a mass spectrometer designed for ESI operation cannot be used for MALDI.